Ink-jet printers have become established as reliable and efficient devices. Typically, an ink-jet printer utilizes a pen mounted on a carriage which is moved relative to a printing surface. Small droplets of ink are formed and ejected from the pen and toward a printing medium. Such pens include print heads with orifice plates having very small nozzles through which ink droplets are ejected by quickly heating a volume of ink within an ink chamber adjacent the nozzle. Ink in the chamber may be heated, for example, using a resistor. This process causes ink within the chamber to super heat and form a vapor bubble. The rapidly expanding ink vapor forces a drop of ink through the orifice.
Recently, innovations in thermal ink-jet printing have concerned ink compositions capable of high-quality color printing using yellow-, magenta-, and cyan-colored ink compositions. These three primary colors, when mixed in various combinations, provide a full range of colors. Black printing may be accomplished by providing a separate black ink or by producing a composite black using the three primary color inks.
Unfortunately, ink compositions that provide improved text print quality may have increased amounts of bleed. More specifically, when inks of two different colors are printed next to each other, they may diffuse into each other, resulting in a ragged border therebetween. Bleed occurs as colors mix both on the surface of the print media as well as within the print media.
Prior approaches to reducing the amount of bleed have included the use of heat sources, such as heated platens, and/or special paper formulations. Heat sources add to the cost and complexity of the printers while inks that require special papers limit the applications for which the printer is useful. Prior approaches to reducing the amount of color-to-color bleed also include adding surfactants to the ink compositions. Surfactants reduce color-to-color bleed typically by increasing the rate inks penetrate the print media. Increasing the ink-composition penetration rate, however, reduces edge acuity.
Moreover, finding a suitable surfactant is difficult. A suitable surfactant modifies the surface energy of the print media so as to increase the ink compositions vertical penetration rate (i.e., the rate at which the ink enters the print media) and reduces the extent of lateral diffusion on the surface of the print media, thereby reducing bleed. The surfactant must, however, possess the appropriate hydrophile lipophile balance (HLB balance). A surfactant having a higher HLB balance value tends to be relatively water soluble and, thus, does not have many of the disadvantages of typical surfactants. Such surfactants also tend to be relatively thermally stable. However, such surfactants also tend to cause more lateral diffusion of ink along the surface of the print media, thereby causing more undesirable bleed. A surfactant having a lower HLB balance value generally, is less soluble in water and, therefore, causes less lateral diffusion of ink along the surface of the print media. Less soluble surfactants, thus, cause less bleed. Unfortunately, less soluble surfactants are also less thermally stable.
More specifically, addition of surfactants, especially those with low HLB values, typically causes ink compositions to be more susceptible to thermally-induced phase separation, which lowers the cloud point of the ink composition. The term "cloud point" refers to the temperature at which phase separation of the ink composition occurs (i.e., the temperature at which the surfactant is no longer soluble in the ink composition). Most ink compositions typically have a cloud point of about 45.degree. C. or lower.
When the cloud point is reached the ink composition has a "cloudy" appearance and multiple phases. Using an ink composition that is at or near its cloud point (or thermally-induced phase separation point) substantially deteriorates the print quality. That is, firing or ejecting ink droplets comprised of an ink composition at or near its cloud point, leads to variable ink droplet velocities and, hence, poor print quality caused by the ink droplets missing their intended targets. Accordingly, in the past, an ink composition including surfactants of low HLB balance values has been desirable to limit bleed, however, such compositions were undesirable due to their lack of thermal stability. An ink composition susceptible to thermally-induced phase separation limits the temperature range over which the composition may be used and stored. Ink compositions having high cloud points (i.e., at least about 60.degree. C. and preferably about 70.degree. C.) are desirable when the ink composition must be stored and/or used in warm environments or climates.
Prior approaches to reducing the amount of black-to-color bleed include, for example, adding one or more salts to the ink composition. Such salts typically include cationic counterions from Group IIA, Group IIIA, transition metals of Group IIIB, and lanthanides, and therefore are relatively toxic. Additionally, such salts tend to cause the ink composition droplets to "decel," which refers to a phenomena that occurs in a nozzle of an ink-jet pen. At rapid droplet ejection frequency the vaporization and ejection of ink droplets of a particular ink composition can suddenly slow or stop completely, i.e., decel. It appears that the reduction in droplet ejection frequency due to decel is a result of a reaction of the salt in the ink composition with the nozzle surface.
Still another important characteristic of ink compositions is the print quality and stability of images printed on "special media," such as starch-coated substrates, silica-coated substrates, clay-coated substrates, and polymeric media, such as transparencies. Of particular interest is the light fastness of the ink composition's printed image on such media. The term "light fastness" refers to the degree the printed image deteriorates due to light and oxidants present in the environment. Many presently available ink compositions do not provide suitable image-quality printing on such media.
A need remains for ink compositions that are resistant to thermally-induced phase separation, do not bleed, and possess other desirable properties, such as a low toxicity level, a relatively long shelf-life, decel resistance, and superior light fastness on special media. Ink compositions are known that possess one or more of the foregoing properties. Few ink compositions possess all of the foregoing properties as improvement in one property often results in degradation of another property, or other properties, of the composition.